Reference for Bava Metzia 14:4

והתניא מצא שטר כתובה בשוק בזמן שהבעל מודה יחזיר לאשה אין הבעל מודה לא יחזיר לא לזה ולא לזה

And [thus we learn that a found bill] must not be returned [to the claimant] not only when it bears no legal endorsement, so that it can be assumed that it was written for the purpose of securing a loan but the loan did not take place, but even when it bears a legal endorsement, as when it has been verified [in Court], because we apprehend that payment may have been made.<span class="x" onmousemove="('comment',' I.e. if the debtor pleads that the debt has been paid, we take this plea into consideration. ');">⁴</span>

Jastrow